Many, if not all, fire codes require certain types of buildings, structures and areas to be equipped with a safety system used to prevent the spread of fires. Such fire protection systems commonly utilize a network of sprinkler pipes for conducting a fire extinguishing medium, such as water, to a plurality of automatic sprinklers which may be arranged vertically, either in a pendent position or an upright position, or horizontally. Two common types of automatic sprinkler systems are the wet pipe type and the dry pipe type.
In wet pipe systems, the sprinkler pipes are filled with a fire extinguishing medium usually water, and connected to an ample supply of the fire extinguishing medium. Individual sprinkler heads are normally closed, but are designed to open, for instance by the melting of an alloy insert, when the ambient temperature reaches a predetermined value, commonly in the neighborhood of 135.degree.-165.degree. F.
If the system piping is subjected to freezing temperatures, for instance, in unheated buildings, such as warehouses, it may be necessary to employ a dry pipe system, rather than a wet pipe system, to prevent the fire extinguishing medium from freezing in the sprinkler pipes. In dry pipe systems, the sprinkler pipes contain a gas, such as air, rather than a fire extinguishing medium. An ample supply of the fire extinguishing medium is connected to the system by a dry pipe valve, which opens in response to the opening of individual sprinkler heads to permit the gas to escape from the sprinkler pipes and water to enter them and discharge from the sprinkler heads.
Dry automatic sprinklers are known which permit the concealment of dry pipe systems and the extension of sprinkler protection to unheated areas from wet pipe systems. Such sprinklers normally have a valve mechanism at the inlet of a nipple which connects an individual sprinkler head to a supply pipe. When the sprinklers are installed in the supply pipe, the valve mechanism extends into the supply pipe, sealing off the nipple until the sprinkler is activated.
Willms U.S. Pat. No. 3,584,689 discloses such a dry sprinkler which includes an outer tube, an inner tube and a cylindrical sealing chamber affixed to the end of the inner tube adjacent a supply pipe and forming a seal with a cap member in the outer tube. In the embodiment shown in FIGS. 1 and 2 of the Willms patent, the inner tube is urged away from an operative position by a compression spring. In the embodiment illustrated in FIG. 3, the inner tube is urged towards its operative position. However, the cylindrical sealing member of both embodiments remains fixed on the end of the inner tube when the valve is released, requiring water to flow axially between the inner and outer tubes and radially through ports in the sealing member and in the inner tube.
The construction of both embodiments of the dry sprinkler of the Willms patent creates two serious problems. First, forces resulting from fluid pressure in the supply pipe are transmitted to the sprinkler head through the sealing member, the inner tube and the compression spring. If excessive, these forces can cause premature activation of the sprinkler. Second, mixed axial and radial flow of fluid through the sprinkler increases the pressure drop experienced by the flowing fluid. Because flow rate descreases as the pressure drop increases, the inner diameter of the inner tube must be selected, i.e., increased, to compensate for the increase in pressure drop, in order to achieve or maintain a desired or required rate of flow through the sprinkler. Providing a larger diameter inner tube increases manufacturing costs, which are further increased by the provision of ports in the inner tube to permit the entry of fluid passing from the outer tube to the inner tube.
In another known type of dry sprinkler, which has been marketed successfully for many years by the assignee of this application and includes an inner tube, an outer tube and an inlet fitting for attaching the outer tube to a supply pipe, a plurality of sealing balls form a fluid-tight seal at the inlet end of the sprinkler. The sealing balls are held in their sealing positions in the inlet fitting by a ring of locking balls which are maintained in engagement with the outlet end of the inlet fitting and an adjacent sealing ball by the inner tube. Upon the collapse of a heat-responsive device located at the outlet end of the sprinkler, the inner tube moves longitudinally, permitting a generally longitudinal movement of the locking balls which, as a result of such movement, disengage the adjacent sealing ball to permit all of the balls to be ejected from the inlet fitting by fluid pressure in the supply pipe.
By this construction, forces resulting from the fluid pressure in the supply pipe are transmitted to the heat-responsive device through the sealing balls, locking balls, and inner tube, creating the possibility of premature activation. Moreover, if the inner tube expands at a greater rate than the outer tube or inlet fitting when the sprinkler is subjected to above ambient temperatures, the sprinkler can be activated prematurely by the exertion of the heat-responsive device of an additional load resulting from the unequal thermal expansion of the tubes and fitting.